Sorry. Cannot really understand the question. What is the context? Why do you think it would be a useful piece of information? - After all every change is different and there are many factors in determining when a change should take place that have nothing to do with efficiency or quickness or slowness (for example a particular change may only be feasible at a certain time of the year, or may be dependent on extraneous factors). I'm having difficlty seeing how it can be genuinely useful information, but perhaps you are really intending to measure time from request to approval to proceed or initiation of planning?

as for how to calculate it, that's simply applying the statistical method for the mean. If you are unclear on this you might get a more reliable answer from a text on statistics (try Wikipaedia, for example) or from a statistician, although I am quite certain that many people here will also know the answer, but less will be experts._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718